Semiconductor wafer holder with CVD silicon carbide film coating

ABSTRACT

A semiconductor wafer holder which has a contact part with a held object to be held is described. The contact part is constructed of a sintered silicon carbide substrate and then coated with a dense CVD silicon carbide film which has at least one crystal plane orientation out of the (111), (110), (220) and (200) planes in Miller indices to provide excellent grindability despite high hardness to withstand abrasive wear, thus facilitating surface grinding of the contact part into an ultra smooth and flat surface without dust and pit holes. This composite material has a large modulus of elasticity, a small specific gravity, and a very low coefficient of thermal expansion, creating high strength and little change in spite of exposure heat and maintaining dimensional stability in circuit printing. The sintered material together with the crystallized CVD silicon carbide film of the β structure offers an electric resistance under 10 10  Ω·cm to provide high conductivity, thereby preventing static dust and facilities cleaning up.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.08/927,024, filed Sep. 10, 1997, U.S. Pat. No. 6,001,180.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a holder for fixing by suction thick-filmsubstrates such as semiconductor substrates, alumina, and quartz.

2. Description of the Related Art

There are various types of conventional holders designed to about asmooth surface of a held object and hold said piece, e.g., devices usedin the manufacturing process of semiconductor IC circuits to carry andfix wafers to preset positions. In exposure systems playing a principalrole in the lithography process, vacuum chuck apparatus are employed tofix thin-film wafers to inside a plane.

It is known that anodized aluminum alloys and alumina ceramics are thematerials currently used for a contact part with the held object to beheld by the holder of this type. It is also known that it is impossiblefor the holder the contact part of which is made of an aluminum alloy tomaintain the accuracy of the section surface thereof over a long periodof time due to abrasion or the like, while the holder having an aluminaceramics-made contact part is also not free from difficulty in that whenin use, the holding part becomes electrically charged attracting dustparticles which transfer and contaminate wafers, diminishing suctionaccuracy of the held objects to be held.

As a result of micro-miniaturization in recent years, insofar as thesemiconductor IC circuit is concerned, it is necessary for the waferexposure surface to have a completely two-dimensionally flat and ultrasmooth surface. Accordingly, it is required that the surface accuracy ofthe suction surface of the holder (vacuum chuck holder) holding the heldobject to be held (wafer) is in ultra smooth finish.

When the contact surface of the holder is shaped by machining, thealuminum alloy, despite being soft and easily machinable, readilydeteriorates. Alumina ceramics tend to absorb dust particles due to theexistence of pin holes and electric charging with a further disadvantageof a large coefficient of thermal expansion.

As explained in the aforementioned prior art techniques, the propertiesrequired of materials of the holders designed to hold the held object incontact with the smooth surface of holders are as follows:

(1) Excellent abrasion resistance, i.e., a high degree of hardness;

(2) No suction of dust particles due to the existence of pin holes andelectric charging;

(3) Small coefficient of thermal expansion;

(4) Low specific gravity and high strength; and

(5) Excellent machinability. The foregoing properties are describedhereinunder in detail.

The contact surface between the holder and the held object wears out andsuffers damage through repetition of the holing operation. Especiallywhen a thin piece such as a thin-film wafer is sucked to a vacuum chuckholder and held there, the surface of the wafer subjected to suctionmust be first corrected to the optimum position for the exposure systemto produce light, that is, as perfect a two-dimensional plane aspossible. Consequently, high abrasion resistance and high hardness arethe desirable qualities of the contact surface between the holder andthe held object to prevent deformation of the piece due to defectscaused by abrasion of the contact surface as well as deformation of thepiece due to presence of minute particles generated by abrasive wearresulting from contact therebetween. In case of wafers, deposits ofminute particles upon the surface result in wire snapping and shortingin circuit printing.

Further, in addition to the aforementioned abrasive wear, dust beingattracted to the electrically charged holder, i.e., infiltrating the pinholes on the ceramic surface, causes the same damage as mentioned above.

In case of the vacuum chuck holder of an exposure system, heat isaccumulated due to the absorption of light at the time of exposure. As aresult, a large coefficient of thermal expansion of the holder bringsabout a dimensional change of the holder proper, making it impossible tomaintain the wafer in the state of being drawn by suction and fixed tothe holder at the preset position. Hence, it is desirable for thecoefficient of thermal expansion to be as small as possible to suppressdimensional errors in circuit printing.

When a large-sized held object is to be held, a load on the holding partof the holder becomes heavy, wherefore a material of high mechanicalstrength is needed for the holder, whereas this would result inincreasing the thickness of the material so that an increase in strengthruns counter to an endeavor to make the holder lightweight.

The most time- and labor-consuming step of the machining process ofmaterials is the grinding step. Particularly, in the case of the vacuumchuck holder for holding wafers, the contact surface with the wafer mustbe ultra smooth because of correction required to attain flatnessthrough vacuum suction. Therefore, grindability of the material of theholder becomes a critical factor.

Ceramic material have lately been developed as the materials meeting theabove-mentioned prerequisite. However, since the existing ceramicmaterials which are generally very hard call for considerable amounts ofmachining as well as grinding energy to effect surface grinding to thelevel of ultra smoothness described above. Intensification of thegrinding work tends to damage the polished surface, hence making itdifficult to obtain a smooth and flat surface of high accuracy.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned problems, an object of the presentinvention is to provide a semiconductor wafer holder which comprises aceramic composite material wherein a silicon carbide film is formed byCVD(chemical vapor deposition) sintered silicon carbide which functionsas a contact part with a held object to be held. As the desirablecomposite material, sintered α-silicon carbide or sintered β-siliconcarbide can be cited for the substrate and crystallized silicon carbideof the β structure can be used for the CVD silicon carbide film.

It is desirable for the above-mentioned CVD silicon carbide film to becomposed of a vapor deposition film having at least one crystal planeorientation out of the (111), (110), (220), and (200) planes in Millerindices. It is further desirable for the said CVD silicon carbide filmto comprise a vapor deposition film having any two or three types of thestructural components of crystal plane orientations out of the (111),(110), (220), and (200) planes in Miller indices. It is most desirablethat the said CVD silicon carbide film includes a structural componentof the (220) plane or orientation in Miller indices and that the saidcomponent has an X-ray diffraction strength exceeding 50 times withrespect to other structural components of crystal plane orientations.

The electric discharge machining method is normally employed to overcomedifficulty in machining conventional ceramic materials. Nevertheless, aregion subjected to electric discharge machining mostly comprises theformation of recesses and pin holes so that for making the contactsurface with the held object smooth, there is no other procedure but torely on stepwise grinding which is the most difficult step of workingceramic materials. It will be appreciated that for the semiconductorwafer holder of the present invention, a sintered silicon carbidesubstrate is first formed and the surface thereof is subsequently coatedwith a CVD silicon carbide film, thus facilitating the manufacturethereof.

Further, inasmuch as the semiconductor wafer holder of the presentinvention does not use the sintered silicon carbide directly for thecontact part with the held object but applies the CVD silicon carbidefilm to the surface which facilitates grinding, the holder excess in themost critical property, i.e., machinability of the contact surface withthe held object into an ultra smooth and flat surface.

Still further, the present invention is characterized by an improvementon the difficulty of surface grinding due to the non-orientation of thecrystal planes on the surface of the conventional silicon carbidematerial by orienting the crystal planes of the silicon carbide vapordeposition film and truing up the planes of cleavage, thus enabling togrind the surface into the ultra smooth and flat surface with lessgrinding energy while preventing the generation of damage as much aspossible.

It will be appreciated that the electric resistance of the sinteredsilicon carbide composite material used for the present invention isunder 10⁶ Ω·cm. This property is a so-called composite possessingconductivity. In other words, in addition to the properties of highrigidity, light weight, and high hardness of the sintered ceramics, thesemiconductor wafer holder according to the present invention hasconductivity which is a property hitherto unattainable in the existingAl₂ O₃ ceramics. Consequently, abrasion resistance thereof has increasedand the generation of the static electricity at the contact part withthe held object is prevented, thereby reducing dust deposit.

The CVD film of this invention is of such dense structure that the filmis free from pin holes which are present in normal ceramic surfaces.Hence, the contact part between the semiconductor wafer holder and theheld object is clear of any deposit of foreign objects on the pin holes,and should there be any deposit, such deposit can be cleaned and removedwith ease.

The CVD silicon carbide film of the present invention can be so producedas to have many kinds of film structures having each orientation planeas mentioned above dependent on the condition of vapor deposition.

It will also be noted that no definite method is designated in formingthe CVD silicon carbide film according to this invention insofar asformation of the film having especially the foregoing function ispossible. Although the normal method is thus employed, formation bymeans of the reduced pressure CVD under non-acidic atmosphere ispreferable.

So long as the semiconductor wafer holder of the present invention has amode of holding the held object by abutting the smooth and flat surfacethereof, no specific limitations are placed on application thereof,e.g., holders (vacuum chuck holders fixing wafers within a plane in theexposure systems of the lithography process, etc.) used for carrying andholding wafers for semiconductor substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features in structure, operation and advantage of the presentinvention will become more apparent to those skilled in the art from thefollowing description when read in conjunction with the accompanydrawings, in which:

FIG. 1 is a schematic plan view of a vacuum chuck holder according to anembodiment of a semiconductor wafer holder of the present invention;

FIG. 2 is a sectional schematic illustration of the vacuum chuck holderof FIG. 1;

FIG. 3 is a partially enlarged view of the sectional schematicillustration of the vacuum chuck holder of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to test examples, the effects of the present inventionwill be described hereinunder in detail.

TEST EXAMPLE 1

There are shown in Table 1 physical properties of vacuum chuck holders(hereinafter may be referred to as "Holders") wherein silicon carbidefilms were formed with each material used for conventional Holders inreduced pressure chemical deposition under non-acidic atmosphere of anembodiment. Each physical property of a metal, ceramics, stainless steelrepresenting respective materials, and alumina are shown for comparativematerials Table 1

                                      TABLE 1                                     __________________________________________________________________________                                    Coefficient                                          Modulus of                                                                              Surface                                                                            Electric  of thermal                                           elasticity                                                                          Specific                                                                          pore resistance                                                                         Hardness                                                                           expansion                                            (kg/mm.sup.2)                                                                       gravity                                                                           condition                                                                          (Ω · cm)                                                            (Hν)                                                                            (×10.sup.-6 /° C.)               __________________________________________________________________________    Stainless                                                                            20,000                                                                              8.1 Medium                                                                             7 × 10.sup.-5                                                                200  17.3                                          steel                                                                         Al.sub.2 O.sub.3                                                                     35,000                                                                              3.8 Poor ≧10.sup.14                                                                  1,800                                                                              7.1                                           Sintered SiC +                                                                       40,000                                                                              3.1 Good 10.sup.8                                                                           2,500                                                                              4.1                                           chemical                                                                      deposition                                                                    SiC film                                                                      __________________________________________________________________________

The following information is obtained from Table 1. As the applicableproperty values for use in evaluation of the holders calling forflatness and the material for carrier devices requiring small size andhigh performance, a comparison of the modulus of elasticity to thespecific gravity indicates that in both cases, the CVD silicon carbidecomposite material according to this invention shows the most desirablevalues of a large modulus of elasticity and a small specific gravity. Ofthe three materials, this material has the least coefficient of thermalexpansion, thus creating little dimensional change even when heat isaccumulated through a long period of exposure time. That is, use of suchmaterial to construct a vacuum chuck holder is certain to produce littledimensional error in circuit printing of a wafer which is fixed bysuction.

There are hardly any pin holes on the surface chemically deposited fromthe vapor or the contact surface with the held object. Hence, thechances are very small for dust depositing on the suction surface,resulting in little possibility of degradation of flatness of the heldobject due to the presence of dust between the contact surface and theheld object. It will also be appreciated that high hardness thereofmakes it impossible to cause minute particles to generate throughabrasive wear and deposit on the held object.

The sintered α-silicon carbide to be used as a substrate shows anelectric resistance of about 10¹⁰ Ω·cm. However, since the carbide has acomposite structure with the crystallized CVD silicon carbide film ofthe β structure according to this invention, the carbide exhibits anelectric resistance under 10⁶ Ω·cm which provides for excellentconductivity in ceramics, hence causing no dust deposit on the holdergenerated by static electricity. Even if there should be some dustdeposit, such deposit can be easily removed. The cleaning up of theholder is therefore simplified.

Further, the small coefficient of thermal expansion is responsible forlittle dimensional change of the holder through heat accumulation at thetime of exposure, etc. Consequently, when the wafer is held and ahigh-sensitive photoresist is used for a long period of exposure timeunder low-intensity light, the dimensional error of the wafer can beheld within the tolerance.

TEST EXAMPLE 2

A comparison between the vapor deposition film obtained by arranging toorient the crystal planes in the (220) and (111) planes in Millerindices and the vapor deposition film of no orientation was made withrespect to the grinding energy required for grinding and time, with afinding that the oriented film was superior to the non-oriented film inthat the oriented vapor deposition film had far less energy consumptionas well as a clear difference in the polished state of surface.

TEST EXAMPLE 3

The crystal structures of both vapor deposition films formed underreduced pressure or normal pressure atmosphere were subjected to X-raydiffraction for comparison, the result being that the crystal structureof the film formed under the reduced pressure atmosphere had the (220)plane approximately 52 more times more than the (111) plane in Millerindices, in contrast to the record of about 32 times more under thenormal pressure atmosphere. Since grinding becomes more difficult in thematerials of non-oriented crystal structure than the materials oforiented crystal structure, this means that the film formed under thereduced pressure atmosphere with more orientations in the direction ofthe (200) plane provides for superior grindability.

EMBODIMENT

An embodiment of a semiconductor wafer holder according to the presentinvention will be described hereunder with reference to the accompanyingdrawings.

FIG. 1 is a plan view showing a schematic illustration of a constructionof a vacuum chuck holder for fixing a wafer which is an embodiment ofthe semiconductor wafer holder according to this invention.

FIG. 2 is a sectional view taken on line A-A' of FIG. 1. FIG. 3 is apartially expanded view of FIG. 2.

A wafer suction surface of the vacuum chuck holder WH according to thisinvention is in a circular shape with a diameter slightly smaller thanthat of the wafer W. A plurality of annular protrusions 1 (wafer Wholding part) and annular recesses 2 (vacuum suction grooves), bothconcentric about the center of a holder WH, are formed in rings at afixing spacing on the wafer suction surface.

At each annular recess 2 is radially formed an inlet port 3 for vacuumsuction, each inlet port 3 in communication with a sleeve-shaped port 4(vacuum suction port) radially extending to inside the holder WH.Connection of the port 4 to a vacuum source with subsequentdepressurization causes a negative pressure in a space surrounded by thereverse side of the wafer W and each annular recess 2, so that thereverse side of the wafer W, following the top surface of the pluralityof annular protrusions 1, is corrected for leveling.

In this embodiment, the holder WH proper was constructed by coating asintered silicon carbide substrate with a silicon carbide film formed byCVD. First, an α-silicon carbide substrate 6 shaped as shown in FIG. 1and FIG. 2 was sintered. Since the sintering frame had already beenworked to provide annular protrusions 1 and recesses 2 (0.2-0.5 mm deep)therein, the formed sintered holder substrate had printed protrusionsand recesses. Machining was again performed to make the crest width ofthe annular protrusions 1 precisely 0.2 mm and grinding was conductedthereon to make a surface coarseness up to 0.01 mm.

The wafer holding surface and the groove surface of the substrate weretreated with simultaneous film forming. Namely, both surfaces of theannular protrusions 1 and the annular recesses 2 were subjected tochemical vapor deposition of high-purity silicon carbide under twodifferent conditions of vapor deposition, resulting in respectiveformations of a silicon carbide film 5. One was depressurized vapordeposition under non-acidic atmosphere with a vapor depositiontemperature of 1300° C. at a vapor deposition rate of 10 to several tensμm/h, while the other was normal pressure vapor deposition undernon-acidic atmospheric with the same vapor deposition temperature at thesame vapor deposition rate.

The two surfaces of the vacuum chuck holder thus obtained were thenmachine-polished by diamond powder for five hours, whereupon the filmsurface obtained by vapor deposition under the aforementioneddepressurized atmosphere was finished by grinding to 8 rms and the otherfilm surface obtained by vapor deposition under the normal pressureatmosphere was finished to 500 rms.

As the foregoing description has shown, inasmuch as manufacture of thesemiconductor wafer holder of this invention comprises a formation of asintered silicon carbide substrate with a subsequent coating of a CVDsilicon carbide film thereon, the manufacture thereof is easy. Also, thesintered silicon carbide is not used directly for a contact part withthe held object, but a CVD silicon carbide film which facilitatesgrinding is used for the surface, thereby providing for excellentworkability of the contact surface with the held object into a smoothand flat surface the most critical factor.

Furthermore, this invention enables the surface of the CVD siliconcarbide film to be ground smooth and flat with consumption of lessgrinding energy while preventing the generation of damage as much aspossible by orienting the crystal planes thereof and truing up theplanes of cleavage. In particular, by coating the CVD silicon carbidefilm under non-acidic atmosphere in reduced pressure, the generated filmforms a specific crystal structure, thereby facilitating formation ofthe ultra smooth and flat surface by means of grinding, which is themost difficult operation in ceramic materials.

Freedom of the silicon carbide film from any pin hole and the propertyof conductivity of the semiconductor wafer holder according to thisinvention prevent dust from depositing due to generation of staticelectricity, wherefore the holder is made simple to clean up.

Another beneficial effect arises from the use of sintered siliconcarbide for substrate: high tenacity brought about by the properties ofhigh rigidity and light weight which are characteristic of ceramicsmakes it possible to hold the held object and make correction to attaina level surface stably over a long period of time. There is a stillfurther property of high dimensional stability as a result of a smallcoefficient of thermal expansion, which is advantageous when dimensionalchanges due to accumulation of heat pose a problem.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A semiconductor wafer holder characterized inthat a contact part of said holder comprises a sintered silicon carbidesubstrate, a CVD silicon carbide film being coated thereon, wherein saidCVD silicon carbide film contains a structural component of the (220)crystal plane orientation in Miller indices, said component having anX-ray diffraction strength in excess of 32 times with respect to theother structural components of crystal plane orientations.